<html>
    <head>
        <script language="javascript" type="text/javascript">
            function aumenta(formulario) {
                teste=formulario.value;
                teste=parseInt(teste)+1;
                formulario.value=teste;
            }
            function diminui(formulario) {
                teste=formulario.value;
                teste=parseInt(teste)-1;
                formulario.value=teste;
            }
        </script>
    </head>
    <body>
        <form name="formalterar" action="teste.html">
        <input type="text" name="mantissa" size="2" value="1" readonly>
        <input type="button" name="aument" value="+" onclick="javascript:aumenta(document.formalterar.mantissa)">
        <input type="button" name="diminu" value="-" onclick="javascript:diminui(document.formalterar.mantissa)">
        </form>
    </body>
</html>